Aircraft manufacturers continuously attempt to improve aircraft performance by reducing both weight and manufacturing costs while maintaining or improving structural strength. One well-known method for increasing aircraft performance is to reduce airframe weight through the use of state-of-the-art materials, such as composites, having relatively high strength-to-weight and stiffness-to-weight ratios. Composite materials are generally described as being materials that include reinforcing fibers, such as graphite fibers, embedded in a polymeric matrix, such as an epoxy resin. Such materials will hereinafter be referenced as “fiber-reinforced composite” materials. Fiber-reinforced composite materials are usually supplied as fibrous sheets pre-impregnated with a curable or partially cured resin. The so-called “prepreg sheets” may then be laid up in laminated plies and cured to form rigid panel structures.
A variety of composite panel structures have been proposed as evidenced by EP 1800842, GB 2110736, U.S. Pat. No. 7,074,474 and U.S. Pat. No. 8,043,554, the entire contents of each being expressly incorporated hereinto by reference.
According to EP 1800842, an elongate structural element and a rigid shell structure integrated with at least one elongate stiffening element are proposed. A preformed, form-stable former is provided having a hat profile with two inwardly directed opposing flange portions, covering the former with a curable composite material to form an external hat profile having outwardly directed flanges. The assembly is integrated with either a curable or preformed shell structure of composite material. One disadvantage of this proposal, however, is that in the event of laminate detachment (peeling), the structure looses its closed section functionality.
GB 2110736 discloses an stiffened panel of fiber reinforced plastics material having a base web, a stringer element in the form of a strip of carbon reinforced plastics material wrapped around a mandrel to define a cavity and having its lower side bonded to the base web.
According to U.S. Pat. No. 7,074,474, a composite material-stiffened panel is provided which has a skin obtained by molding a fiber-reinforced resin composite material into a flat skin and stiffeners arranged in rows on one surface of the skin. A fiber-reinforced resin-composite material is stitched on the skin covering the stiffeners.
U.S. Pat. No. 8,043,554 describes structures having composite hat stringers. The structures are fabricated by placing prepreg layers into a mold tool base to form an exterior hat portion of the stringer. An inflatable bladder may be used during the curing process so as to create compressive forces in conjunction with the mold tool to compress each of the wall portions of the stringer.
While the various proposals noted above are suitable for their intended uses, there is still a need for improvements. It is therefore toward providing improvements to the integrated composite structural panels and processes for fabricating the same as known in the art that the embodiments of the present invention are directed.